For a working vehicle such as a construction machine or the like, techniques have been known from the past of changing over the power output performance of an engine according to the work load (for example, see Patent References #1 and #2). According to these techniques, the working vehicle is provided with a heavy work mode in which it is possible to obtain power output up to a high level, and with a light work mode in which it is only possible to obtain power output up to a low level.
The driver selects between these modes by hand by operating a changeover switch. In other words, if he determines that the work which will be performed henceforward is heavy work, then he selects the heavy work mode, while if he determines that it is light work, then he selects the light work mode.
At this time, a controller which controls the engine controls the power output capacity of the engine according to a command from the changeover switch. In other words, for the light work mode, it limits the power output range of the engine to be less than or equal to a predetermined value which is lower than its rated power output, for example by limiting the fuel supply amount. By contrast, for the heavy work mode, it does not impose the above described limitation, so that it is possible for the power output of the engine to reach the rated power output or the maximum power output.
Since, by doing this, a narrow low power output range is used during light work, accordingly the consumption of energy becomes small so that the fuel consumption is reduced. And since, during heavy work, no limitation is imposed upon the power output of the engine, it is possible to obtain sufficient power output for performing the work smoothly.    Patent Reference #1: Japanese Patent Laid-Open Publication Heisei 08-218442.    Patent Reference #2: Japanese Patent Laid-Open Publication Heisei 11-293710.
With a certain type of working vehicle, it often happens that the vehicle does not continuously perform only heavy work or only light work, but rather performs heavy work and light work alternately during a series of work processes. For example, with a wheel loader, during typical excavation and loading work, the vehicle successively performs processes such as approaching the subject material for work (which is light work), excavating the subject material for work and loading it into the bucket (which is heavy work), and dumping of the subject material for work upon a transportation vehicle such as a dump truck or the like (which is light work).
With this type of working vehicle or working process, in order to enjoy the benefits of the prior art technique to the maximum level, the driver must operate the changeover switch while changing over between heavy work and light work. However, it is very burdensome to perform such switch operation frequently during the working process. As a result, it often happens that work is performed with the changeover switch always left fixed in the heavy work mode, so that it is not possible to anticipate any reduction in the fuel consumption. On the other hand, if emphasis is placed upon the fuel consumption, and work is performed with the changeover switch left fixed in the light work mode, then it is not possible to obtain sufficient power output during heavy work such as excavation, and there is a fear that the working efficiency will be deteriorated.